TGF-Beta and related polypeptides regulate cell growth, differentiation, motility, organization and death. Some of them participate in setting up the basic body plan during early embryogenesis in mammals, frogs and flies, whereas others control the formation of cartilage, bone and sexual organs, suppress epithelial cell growth, foster wound repair, or regulate important immune and endocrine functions. The TGF-Beta family is therefore viewed as one of the most influential groups of growth and differentiation factors. Elucidating their mode of action should unveil regulatory mechanisms of considerable interest. The overall goal of this project is to elucidate the mechanisms that initiate TGF-Beta signal transduction. Under the auspices of this grant over the past 12 years, we have isolated various TGF-Beta isoforms and shown their role as major regulators of cell proliferation, differentiation and extracellular matrix formation. We have identified various types of membrane receptors for TGF-Beta that participate in signal transduction, or ligand presentation to signaling receptors. We have cloned receptor cDNAs, generated a genetic model system to study the activity of these receptors, and demonstrated their role as mediators of antiproliferative and transcriptional responses to TGF-Beta and related factors. We now propose to investigate the mechanism of TGF-Beta receptor activation, signal specification and signal propagation. Our recent evidence suggests that the primary TGF-Beta receptor (called 'type II' receptor) acts as a constitutively active transmembrane serine/threonine kinase. Upon binding ligand, this receptor recruits and phosphorylates a second, distantly related, transmembrane kinase ('type I') that then propagates the signal to downstream substrates. We plan to test and characterize this model in detail, and identify TGF-Beta receptor substrates and interacting proteins involved in signal propagation. In addition, we propose to investigate the regulation of TGF-Beta access to signaling receptors in vivo by the type III receptor betaglycan. Betaglycan can regulate, both positively and negatively, TGF-Beta access to membrane receptors in cell culture. Since TGF-Beta is a prototypic antimitogenic cytokine, its mode of action may show us ways to deter the runaway proliferation of the cancer cell.